I think we need to double check the goals here
One of them is you want get back control of your PC
--- Can you move the computer to a safe location so that the intruder cannot get to it anymore so you can work on getting control of your computer without further intrusions?
Wait a minute, this can be very important:
--- If you remove the MB battery and reinstall it after a 5 or 10 minutes, that might clear the setup password which hopefully help you move on with following projects.
--- Another thing that might work is to jumper your BIOS settings on the MB and then return the jumper settings afterward back to where they belong . It's been a long time since I have done that so I'd have to review the correct way to do so
--- Ask for help on this if the MB battery idea doesn't work
--- Someone else can certainly chime in on that but I think it will reset your BIOS to its defaults
Here are more ideas to consider which I had typed in before thinking of the MB battery and/or jumper settings and I really don't want to remove what I've already typed in
Can you get into Safe Mode?
--- If so you can remove any other users on your computer but make sure you assign a very strong password to each of your accounts: your Safe Mode Administrator account and your log on account
--- Have you checked out any password finder programs?
------ I don't know whether or not it will help find the setup password, but then the choice you need to make is just get into reinstalling your complete system, but as already noted you will need to reinstall all updates and applications
Another one is you don't have the correct discs to attempt a repair or reinstall your OS
--- Have you contacted Dell about getting the exact disc(s) for your computer?
--- You can also go to their website and download the correct drivers for your unit
--- That's the first option I would try
Run www.belarc.com to get the report of your computer so you will have pertinent numbers available and print out the report just in case you need such info in the middle of doing some thing
Get the numbers on the tower, side back wherever they are
Dell should be able to help you
--- See if you go on-line to order the disc(s). I'd say in todays world it should be somewhere between $20.00 - $30.00
--- If you can't you can try sales but it will cost more but if you need to do something to get back control of your computrt
Whatever other disc(s) you have, label what they are for or what you think they are for if you want to keep them or get rid of them
You can always contact www.restoredisks.com to see if they can help you
You did not have a chance to back up some My Docs or My pics items to CD
--- Not knowing how far back this issue goes, have you tried System Restore?
--- Consider www.recuva.com I haven't had to use it but if you want to try it, ask for help
--- If neither of those ideas are good for you, you will have to move on to get your other projects done
Finally you mention that you think you have security issues and that certainly can be so
--- But if you need to wrestle control from someone else that should take first priority
--- Without being able to clean out their passwords and user accounts your solution is pretty limited: reinstall with the correct Dell discs(s) etc